1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image on a recording material. As such image heating apparatus, mention can be made, for example, a fixing apparatus for fixing an unfixed image on a recording material, a gloss improving apparatus or the like for heating an image fixed on a recording material to thereby improve the gloss of the image. Also, such an image heating apparatus is used, in an image forming apparatus such as, for example, a copying machine, a printer, a facsimile apparatus or a compound machine provided with a plurality of functions of these.
2. Description of the Related Art
Heretofore, in an image forming apparatus of an electrophotographic printing method, a technique of forming a toner image on a recording material such as paper by a technique called the Carlson process, and thereafter fixing the toner image as a permanent image has been popular.
As fixing methods therefore, various methods have been proposed, but from the viewpoint of fixability, a method of heating and fixing a toner image (heat-fixing method) is popular. Above all, a method of directly bringing a toner image into contact with a rotary member containing a heating source therein to thereby fix the toner image is widely used.
In this heat-fixing method, it is important to uniformize the temperature distribution or a heating rotary member such as a roller or film, including an area through which a recording material (hereinafter referred to as the paper) passes, relative to an axial direction. For if there is a portion in which the temperature is lower than a predetermined temperature, there is the possibility of faulty fixing occurring, and on the other hand, if the temperature is too high, there is the possibility of the heating rotary member or a member proximate thereto receiving thermal damage. Further, if the temperature of a non-sheet passing portion has become too high as compared with the temperature of a sheet passing portion, the temperature of the end portion of the sheet passing portion becomes too high as compared with a proper fixing temperature, and this gives rise to the fear that hot offset should occur.
In recent years, there is a demand for an image forming apparatus which copes with various paper sizes from paper of a relatively large size such as, for example, A3 size to paper of small sizes such as A4R and B5 size usually used. Therefore, it is necessary to construct the axial lengths of the heating rotary member and a pressure rotary member so as to correspond to a relatively large size such as, for example, A3 size. However, in a case where the construction as previously described is adopted, when paper of a small size such as A4R or B5 passes through a fixing apparatus, a non-sheet passing area through which the paper does not pass increases in the effective fixing area of the heating rotary member. When copying is continuously effected on the paper of a small size, heat is not taken away from the surface of the heating rotary member corresponding to the non-sheet passing area by the paper and therefore, the surface temperature of the non-sheet passing area becomes very high.
In order to solve the above-noted temperature rise of the non-sheet passing portion, the following propositions have been made.
Conventional Example 1: the supply of heat to the heating rotary member is stopped between sheets, and idle rotation or the like is effected so that the surface temperature of the heating rotary member in the non-sheet passing area may become the same as the surface temperature of the sheet passing area to thereby cope with the problem.
Conventional Example 2: there is adopted a technique of changing the light distribution ratio of heating means such as a heater contained in the heating rotary member so that the quantity of heat supplied to the non-sheet passing area may be smaller than the quantity of heat supplied to the sheet passing area for effecting the fixing of paper of a small size.
The above-described conventional examples, however, suffer from the following problems. In Conventional Example 1 described above, during the continuous supply or the like of small size paper, idle rotation for the cooling of the heating rotary member is necessary between sheets, and this gives rise to the problem that the productivity when the paper of a small size is supplied becomes bad. In recent years, a user's requirement for the productivity of the apparatus has gradually become higher, and it may lead to the possibility of becoming unable to satisfy the product specification required by the user to lower the productivity during the mixed loading of various sizes.
Next, when a construction like Conventional Example 2 is adopted, to cope with a plurality of paper sizes, it is necessary to dispose a heater having a plurality of light distributions, and this leads to the fear of an increased cost.
Also, in order to prevent the rise of the surface temperature of the non-sheet passing area during the supply of small size paper, there has been proposed a construction for cooling the non-sheet passing area by cooling air, as described in Japanese Patent Application Laid-open No. S60-136779 and Japanese Patent Application Laid-open No. H05-181382.
In a fixing apparatus described in Japanese Patent Application Laid-open No. S60-136779, the surroundings of a pressure roll is partitioned into a sheet passing area side and a non-sheet passing area side by a partition plate, and cooling air is blown from a cooling fan disposed in the interior of the fixing apparatus to the outer peripheral member of the pressure roll on the aforementioned non-sheet passing area side.
In Embodiment 1 (FIG. 21) described in Japanese Patent Application Laid-open No. H05-181382, a cooling fan 4 is disposed above a top plate 5 covering the upper part of a fixing roll 1; and normally air is blown to the upper side of the top plate 5 to thereby prevent the temperature rise around a fixing device. When paper passing through a fixing area is small size paper, a window 6 as a guide device provided in the top plate 5 is opened to thereby let cooling air flow to the surface portion of the fixing roll 1 rotated through the non-sheet passing area.
However, the apparatuses described in Japanese Patent Application Laid-open No. S60-136779 and Japanese Patent Application Laid-open No. H05-181382 suffers from the following problems.
In the conventional examples wherein cooling air is blown to the heating rotary member, a cooling fan is provided in the interior of the fixing apparatus and therefore, it is necessary to use a cooling fan of high heat resistance, and this leads to the fear of an increased cost. Also, a cooling fan having a relatively great air flow amount becomes necessary, and the fixing apparatus itself becomes bulky. Also, there is the problem that even if a partition plate is provided, cooling air flows from the non-sheet passing area side into the sheet passing area, and the temperature is lowered near the boundary between the sheet passing area and the non-sheet passing area and the fixing temperature of the partition boundary portion becomes low to thereby cause faulty fixing.
As means for solving the above-noted problem, a fixing apparatus of the following construction has been proposed. That is, the fixing apparatus is provided with a cooling device having a non-sheet passing area cooling duct having an air blowing port formed in opposed relationship with the surface of the non-sheet passing area of a heating rotary member in order to cool the surface of the non-sheet passing area of the heating rotary member, and an air blowing fan for blowing cooling air to the duct. It has a shutter (closure plate) for adjusting an area for blowing the cooling air to the abovementioned non-sheet passing area to an optimum width in accordance with the width of small size paper. The fixing apparatus further has an opening width adjusting member for adjusting the opening width by the shutter in accordance with the width of the aforementioned small size paper.
Also, the cooling fan for blowing the aforementioned cooling air has control means for switching on and off the cooling fan at predetermined timing by the use of the result of the temperature detection of the aforementioned non-sheet passing area. Further, the shutter for adjusting the area blowing the aforementioned cooling air to an optimum opening width in accordance with the width of the small-sized sheet shields the heating rotary member and the cooling fan. Therefore, there can be provided a fixing apparatus in which the temperature of the cooling fan portion does not rise, and which can efficiently prevent temperature rise at a low cost/saved space without using a cooling fan of high heat resistance, and is also excellent in safety.
The above-described proposition, however, suffers from the following problem. In a case where a print job in which paper of a large size and paper of a small size are mixedly prevent is executed, if as shown in FIG. 22 of the accompanying drawings, the changeover operation of the position of the shutter is performed each time the paper size changes between the large size and the small size, there will arise the problem that the life of a shutter driving portion becomes short. Also, if the movement of the shutter is carried out at a high frequency, it will lead to an increase in the electric power consumption of the apparatus.